The ventilatory response to CO2 and hypoxia in the hypocapnic zone, is being studied to elucidate the contributing processes which determine a flat (the characteristic which is commonly called a "dog leg") as contrasted with a steep response (which may be brought out in exercise or by hypocapnic conditioning). The hypotheses incorporate co-existing excitatory and inhibitory mechanisms which originate in chemical and neural processes. Continuation studies of long-term respiratory adaptation to chronic hypoxia (simulated high altitude) are being conducted in rats and cats. The tentative hypothesis that the ponto-medullary ventilatory control system is subject to descending facilitatory and inhibitory influences will be explored further by seeking correlates of characteristic response with appropriate electrical and chemical events in cortical and mid-brain regions. BIBLIOGRAPHIC REFERENCES: L-c. Ou: Intravascular hemolysis in rats maintained at high altitude. Fed. Proc. 35:238, 1976. C.P. Black, S.M. Tenney and A.R. Paulsen: Hypoxic tolerance and ventilatory response in the bar-headed goose. The Physiologist 19:130, 1976.